In a technique disclosed in JP-A-2003-219675, a power supply current sensor is provided in a power supply path from a battery (i.e., a power supply) to a drive circuit which drives an electric load such as a motor. When a short-circuit failure occurs in the drive circuit, the power supply current sensor detects an overcurrent condition, and a power supply relay is turned OFF so that the power supply path can be interrupted.
When a power supply current sensor is used to detect an overcurrent condition, a detection error may be caused, for example, by noise or a short circuit due to a momentary circuit contact. To eliminate such a detection error, a repetitive monitoring process may be performed. In the repetitive monitoring process, a power supply relay is repeatedly turned ON and OFF after an overcurrent condition is detected once, and then when the overcurrent condition is detected a predetermined consecutive number of times, it is finally determined that a short-circuit failure occurs. For a drive circuit with one system having an inverter, this repetitive monitoring process is effective at eliminating the detection error, thereby preventing an unnecessary fail-safe operation from stopping functions.
For example, in an electric power steering system for a vehicle, multiple systems, each of which has a motor drive circuit, are configured in a redundant manner to increase reliability. In general, an input stage of a drive circuit such as an inverter is provided with a capacitor to smooth an input voltage. In an apparatus where a drive circuit of each of multiple systems is connected in parallel to a power supply, a capacitor of an input stage of each drive circuit remains charged during normal operation.
Here it is assumed that a power supply relay is turned OFF when an overcurrent condition in a drive circuit of one of two systems is detected once. The system where the overcurrent condition is detected once is sometimes hereinafter referred to as the “temporary abnormal system”, while the other system is sometimes hereinafter referred to as the “normal system”. If the drive circuit of the temporary abnormal system is actually short-circuited, the capacitor of the input stage of the temporary abnormal system is discharged. For this reason, when the power supply relay is turned ON again in the repetitive monitoring process, charges stored in the capacitor of the input stage of the normal system flows as an inrush current into the temporary abnormal system. Therefore, even when the short-circuit failure is cured so that the temporary abnormal system returns to normal, the repetitive monitoring process incorrectly determines that the temporary abnormal system remains in an overcurrent condition due to the inrush current. That is, the inrush current is incorrectly detected as an excessive power supply current.
Further, in the normal system, when the power supply relay is turned OFF upon the incorrect determination that the temporary abnormal system remains in an overcurrent condition, a secondary inrush current flows from the power supply to the capacitor of the input stage. Due to this secondary inrush current, it may be incorrectly determined that the normal system is in an overcurrent condition. Therefore, in an apparatus with multiple systems, each of which has a drive circuit, it is difficult to finally determine that the temporary abnormal system is in an overcurrent condition by the repetitive monitoring process.